1. Field of the Invention
The present invention is related to a scanning electron microscope (SEM), more particularly, to a scanning electron microscope incorporating a unit for reducing image obstacle caused by external disturbances.
2. Description of the Prior Art
The scanning electron microscope (SEM) is, in general, an apparatus for observing an enlarged image of the surface of samples by emitting an electron beam from an electron beam gun, collimating the beam with electronic lenses, and scanning the sample surface with the collimated electron beam by means of a deflector, in order to form image signals by detecting secondary electrons such as secondary or reflected electrons generated from the sample by the electron beam irradiation.
In case that the SEM apparatus is installed in a location of fluctuated external environment, the electron beam may be improperly deflected by external disturbances so that the scanning image will be disarranged. Some typical examples of the external disturbances resulting in the disarranged scanning image include, for example, mechanical vibration and alternative magnetic fields penetrating from the surrounding environment. In order to overcome this problem the conventional SEM has means such as an antivibration structure or a magnetic shield with a microscopic housing. However the effect of such conventional means is limited. Due to recent higher performance (highly magnifyable) apparatus with the extension of the applicable fields being wider, more serious cases arise in which image problems occur due to the installation environment of the microscope.
In order to achieve the solution of these problems, for example for the mechanical vibration, a large vibration insulator is mounted on the floor of the microscopy room, and the entire microscope system is installed thereon. For the alternative magnetic field, the entire microscope system may be enclosed within a large iron box for shielding the magnetic field, or large coils may be installed in the X and Y direction in the space surrounding the scanning electron microscope apparatus to generate inverted magnetic fields in synchronism with the incoming external magnetic fields by supplying electric current to the coils in order to cancel out the external fields.
For alternative magnetic fields, in general, when taking photographs of the image, by synchronizing the scan of the electron beam with the power supply, the synchronized electron beam scanning with the fluctuation of the electron beam affected by the external alternative magnetic fields may prevent the image from blurring.
Such means for reducing image obstacles as described above require for the SEM to be installed in a specially designed and constructed room Thus the apparatus may not be used where it is needed to be used, and its cost may be extremely high.
When synchronizing the electron beam scan with the power supply frequency, no blurred image may result from the effect of the external alternative magnetic fields. However, the raster of the electron beam may miss the original straight beam line to, for example, draw a curved trajectory such as sinusoidal raster trajectory. The image thus formed and arranged on a monitor screen may or may not indicate correctly the accurate and precise structure of the sample, and may be distorted.